Smaller tires would help, but it may not be necessary. For just about any 1 piece concave rim, the concavity is determined by the wheel width. Offset has nothing to do with it since they change the offset by adding or taking away material from the backpad (the part that mates with the hub). They basically just cast or forge 1 shape per width and then simply machine material off the backpad. So if concavity is your concern, just get the 10.5" wheels in a higher offset like +45 and then play with spacers to get the look you want. I will look identical to buying a lower offset wheel without spacers.
I'm running 20x10.5" wheels on my 1st gen. I run smaller tires 275/40 which are smaller in diameter. But my car is slammed and I'm running +5 in front and +0 offsets in rear, which are much more aggressive than most people would opt for. I have to run smaller tires for clearance, it was my last option after rolling my fenders as much as they will roll. If you don't have to run smaller tires, don't. It looks a bit funny if you're not slammed. There's a post of a 2nd gen, the guy who got the Niche wheels and he's running 265's on a 10.5" wheel, they look tiny.
